You may also like

Mind Reading

Think of a number, add one, double it, take away 3, add the number you first thought of, add 7, divide by 3 and take away the number you first thought of. You should now be left with 2. How do I know?

How Many Miles to Go?

How many more miles must the car travel before the numbers on the milometer and the trip meter contain the same digits in the same order?

Summing Consecutive Numbers

15 = 7 + 8 and 10 = 1 + 2 + 3 + 4. Can you say which numbers can be expressed as the sum of two or more consecutive integers?

Your Number Is...

Age 7 to 14
Challenge Level

Press the button in the interactivity below, and see what happens:

Now try again starting with a different number.

Try again.

Try starting with a fraction... or decimal... or negative number...

Why are your answers ALWAYS the same?

Can you design instructions for similar "Think of a number" machines of your own?
Test them on a partner to check that they lead to your anticipated solutions.
You may want to take a look at Your Number Was... for a suitable follow-up activity.